Titanium is a kind of very important strategic material and plays a vital role in global economy. China is one of few countries rich in titanium reserves and the Panxi area holds more than 90% of the national reserves. However, ilmenite in this region is mainly intergrowth ore. Its titanium dioxide grade is low, normally around 46-47 wt %. Secondly, ilmenite (also called ilmenite concentrate) in this region has high calcium and magnesium impurity contents and normally the calcium oxide and magnesium oxide content is approximately 5-7% in total. In addition, fine granularity is another dominant characteristic of the ilmenite, about 50% of which is less than 200 mesh. Furthermore, the ilmenite ores get finer as mining more deeply.
Nowadays, the two main processes worldwide of preparing titanium pigment are chloride process and sulfuric acid process.
The sulfuric acid process utilizes primary ilmenite or titanium slag to prepare titanium pigment by digesting, leaching, (concentrated crystallization), hydrolysis, water-wash, bleaching, calcination with crystal seed added and other technics. The titanium slag used wherein is obtained by pyrometallurgical smelting: ilmenite together with reducing agent is heated in electric furnace at 1600-1800° C., the ferriferous oxide in ilmenite is reduced into metallic iron by reducing agent at high-temperature smelting, and then the molten iron is separated from slag and get the product of pig iron, which enables most of iron ingredient removed from the ilmenite while titanium and other impurities remain in the smelting slag to form titanium slag. Although it results in heavy pollution, the sulfuric acid process is predominantly used in preparing titanium pigment in Panxi area and even in China due to mature techniques, low requirements of feed stock and low cost.
The chloride process causes much less pollution than the sulfuric acid process but it needs higher-quality feed stock, which usually require higher titanium dioxide content than 90%, bigger granularity than 200 mesh, and low impurity content. Thus, the chloride process mainly utilizes rutile as feed stock, especially synthetic rutile, since the reserves of natural rutile are extremely low, only 0.3-1% of the total reserves of titanium. There are difficulties in preparing synthetic rutile using Panxi ilmenite due to its low ore grade, high impurity and fine granularity. It was indicated in the article entitled “Ilmenite Hydrochloric Acid Leaching for Synthetic Rutile Preparation” (Nonferrous Metals, 59 (4), November 2007, pp. 108-111) that hydrochloric acid leaching is the preferred method to prepare synthetic rutile, after comparing the advantages and disadvantages of different techniques and considering the features of Panxi ilmenite. Currently, two major technical processes of hydrochloric acid leaching are Preoxidation-Liquefaction Leaching and united technologies of beneficiation and metallurgy Pressure Leaching. Preoxidation-Liquefaction Leaching resolved the particle size change problems during the raw ore leaching process through preoxidation. However, the fining rate of this process is still 15% approximately (“Research on Preparing Technology of Synthetic Rutile by Hydrochloric Acid Leaching”, IRON STEEL VANADIUM TITANIUM, 27 (2), June 2006, pp. 1-6). Given the fact that more than 50% of ilmenite concentrate is less than 200 mesh, at least about 60% of the synthetic rutile is less than 200 mesh. Thus, it is of little economical benefits to prepare titanium pigment by utilizing synthetic rutile obtained from Panxi ilmenite following the state-of-the-arts.
The chloride process requires the feed stock containing more than 90% of titanium dioxide, which usually is the synthetic rutile prepared by hydrochloric acid leaching. In the past years, technicians in the production and research field of titanium pigment have been dedicated to solve the problems of granularity and leaching behavior during the hydrochloric acid leaching process for Panxi ilmenite, so as to upgrade the quality of synthetic rutile which is then utilized to prepare titanium pigment by chloride process. But none have ever utilized the intermediate product, that is the titanium residue obtained by the process of “acid leaching—filteration—washing” (described as enriched titanium hydrochloric acid extract residue in the present invention), in the production of titanium pigment, not to mention that anyone has utilized the enriched titanium hydrochloric acid extract residue to prepare titanium pigment by sulfuric acid process.